Heretofore precise positioning with a GPS survey instrument has not been effective in areas where some or all of the signals from the GPS satellites are disrupted. This problem has several dimensions that include a technical dimension, an economic dimension and a man-machine interface (MMI) dimension.
The economic dimension is the GPS survey instrument's value proposition that justifies its price of $35K-50K. The value proposition is the significant improvement in efficiency over alternative precise positioning methods that include conventional total stations (CTS), automatic total stations (ATS) and fan lasers. A GPS survey instrument requires one operator and can operate effectively over an area of up to 10 km away from a companion base receiver providing position correcting information. A CTS selling for around $10K requires a crew of two operators and has an operational range of a few hundred meters. An ATS selling for around $45K requires only one operator, but has a range limit similar to a CTS. In addition, the CTS and the ATS each requires a fairly elaborate setup per location. Fan lasers have even shorter range limits and require elaborate and time-consuming installations. Consequently, under normal circumstances, the GPS survey instrument can provide an excellent value proposition so long as it delivers reliable centimeter-level positioning needed for most survey-grade applications.
Unimpeded GPS survey instruments alone are normally able to provide surveyed position accuracy competitive with CTS, ATS, fan lasers, or traditional rod-and-chain instruments. When aided by differential or RTK data, GPS accuracy can be on the order of one centimeter (cm) for precision land survey. GPS accuracy can vary from 10 cm to one meter for lower accuracy survey applications such as cadastral survey 5, geographic information system (GIS) and seismic survey.
The accuracy of a GPS survey instrument diminishes when one or more satellite signal lines of sight pass through foliage. For example, the current generation GPS survey instrument is not reliable near trees or buildings that can shade, reflect or refract the GPS satellite signals. Such an area is hereinafter referred to as a GPS precision-denied zone. In other words, a GPS precision-denied zone is an area or region where GPS accuracy in locating a point in three dimensional space is degraded. For example, GPS accuracy may degrade from 1 cm to 3 cm in a precision land survey due to signal refraction from nearby foliage or buildings. Accordingly, although a GPS receiver may continue to provide a position solution while within a GPS precision-denied zone, it cannot reliably provide a precise survey-grade position solution having, for example, centimeter-level accuracy. If an operator foresees the need for a CTS or ATS as frequent backup because of extensive foliage in a job area, it is likely that a CTS or ATS will be selected for use on the entire job, and a GPS survey instrument would not be used. The value proposition of the GPS survey instrument thus diminishes in the presence of foliage or other satellite signal obstructions.
What is needed therefore is a technical solution that is able to provide survey-grade precision data by which objects or targets within GPS precision-denied zones may be accurately located.